Problem with dynamic change a page item value

Question

Hi,
I'm trying to dynamically calculated the value of an item based on a simple formula that involves the multiplication of two elements from the same page.
To do this, create dynamic action as follows:
Event: Change
Selection Type: Item(s)
Item(s): P19_DURATION
Action: Set value
Fire on page load: TRUE
Set Type: SQL Statement
SQL Statement: SELECT :P19_DURATION * :P19_RATE FROM DUAL
Page items to submit: P19_DURATION
Selection Type: Item(s)
Item(s): P19_AMOUNT
When altering the P19_DURATION get no change in P19_AMOUNT
Since my knowledge of javascript is almost zero, I'm trying to do this with pl / sql or sql. Try the dynamic action with a function or a PL / SQL statement and nothing works.
Have to use javascript for this?
Searching the forum I think it's something simple, but I did not find an example similar to what I need.
Regards

Answer 1

Event: Change
Selection Type: Item(s)
Item(s): P19_DURATION
Action: Set value
Fire on page load: TRUE
Set Type: javascript
javascript Code:
amount = $v('P19_DURATION') *  $v('P19_RATE');
$s('P19_AMOUNT',amount);